Reduction of MEMS mirror edge diffraction in a wavelength selective switch using servo-based rotation about multiple non-orthogonal axes
View Patent ↗Effects of diffraction of a spectral beam from an edge of the micromirrors are reduced in order to optimize the passband in a wavelength selective switch. The effects of diffraction on the pass band may be reduced by using rotation of the micromirror about both the attenuation axis and the switching axis to achieve the desired level of attenuation. Peak coupling can be attained by dithering the micromirror about a dither axis that is tangent to a contour of constant attenuation using simultaneous rotation about the switching and attenuation axes. A power level of a spectral channel may be attenuated by rotating the channel micromirror with respect to an effective attenuation axis that is non-orthogonal to the dither axis through a combination of rotations about the switching axis and the attenuation axis.
1. Optical apparatus for switching multi-channel optical signals having spectral channels of different wavelengths, comprising:
a plurality of input and output ports for optical signals having one or more of said spectral channels;
an optical beam expander and relay system adapted to receive the optical signals from one or more of the input ports, the optical beam expander and relay system being formed to convert the optical signals to spectral beams having a predetermined elongated beam profile;
a wavelength separator for spatially separating the spectral beams into constituent spectral channels; and
an array of channel micromirrors, each channel micromirror of the array being positioned to receive one of said constituent spectral channels, the micromirrors being rotatable about a switching axis y to switch said one spectral channel to a selected output port;
wherein each channel micromirror is rotatable about an attenuation axis x to vary coupling of said one spectral channel to the selected output port to control a power level of the spectral channel output at such selected port, wherein the attenuation axis is different from the switching axis,
wherein each channel micromirror is configured to dither with respect to a dither axis y′ that is substantially tangent to a contour of constant attenuation by rotating simultaneously about the switching axis y and the attenuation axis x, wherein the dither axis y′ is non-orthogonal to an effective attenuation axis x′, and
wherein each channel micromirror is configured to attenuate the power level by rotating the mirror with respect to the effective attenuation axis x′ through a combination of rotations about the switching axis y and the attenuation axis x,
wherein rotating the mirrors about the dither axis y′ and the effective attenuation axis x′ reduces an increase in signal intensity of a spectral channel near side edges of a passband for that spectral channel relative to a central portion of the passband due to diffraction of that spectral channel from an edge of that spectral channel's associated micromirror, wherein the edge is substantially parallel to the attenuation axis.
2. The apparatus of claim 1 wherein rotations about the dither axis y′ and the effective attenuation axis x′ are related to rotations about the switching axis y and the attenuation axis x through a rotation matrix A of the type:
A
=
[
cos
θ
-
M
sin
θ
(
sin
θ
)
/
M
cos
θ
]
wherein θ is an angle that is not equal to 0° or 90° or some integer multiple of 90° and M is not equal to 1.
3. The apparatus of claim 2 , wherein the value of M is a ratio of an x-axis optical magnification to a y-axis optical magnification of said one spectral channel.
4. The apparatus of claim 1 wherein the optics include an anamorphic system configured to convert the optical signals to spectral beams having a predetermined elongated beam profile.
5. The apparatus of claim 1 , further comprising an array of biaxial port micromirrors for maintaining optimum coupling efficiency into each port and maintaining optimum ITU grid alignment.
6. The apparatus of claim 1 , further comprising means for implementing hitless switching of the spectral channels.
7. The apparatus of claim 6 , wherein the means for implementing hitless switching includes means for substantially attenuating or blocking one or more of the spectral beams during switching.
8. The apparatus of claim 7 wherein the means for substantially attenuating or blocking one or more of the spectral beams during switching includes:
means for disengaging a servo control loop controlling the switching axis of channel micromirror;
control electronics adapted to apply a control voltage to the channel micromirror to rotate the micromirror about its attenuation axis by an amount required for blocking while the servo control loop is disengaged;
means for rotating the channel micromirror about its switching axis to reflect an input spectral channel to a first output port;
means for rotating the micromirror about its attenuation axis back to a normal optimal coupling condition, and
means for re-engaging the servo control loop controlling the switching axis to control the micromirror to achieve optimum coupling efficiency.
9. A method of optimizing passband in optical apparatus for switching optical signals of a multi-channel optical signal having constituent spectral channels of different wavelengths between input and output ports, comprising:
spatially separating said multi-channel optical signal from one of the input ports into spectral beams corresponding to the constituent spectral channels;
focusing the separated spectral channels onto corresponding channel micromirrors that switch the focused spectral channels to one or more selected output ports; and
dithering a particular channel micromirror with respect to a dither axis y′ that is substantially tangent to a contour of constant attenuation through a combination of rotations about a switching axis y and an attenuation axis x, and
attenuating a power level of a spectral channel by rotating the particular channel micromirror with respect to an effective attenuation axis x′ through a combination of rotations about the switching axis y and the attenuation axis x, wherein the dither axis y′ is non-orthogonal to the effective attenuation axis x′,
whereby rotating the particular channel micromirror about the dither axis y′ and the effective attenuation axis x′ reduces an increase in signal intensity of a spectral channel near side edges of a passband due to diffraction of that spectral channel from an edge of that spectral channel's associated micromirror, wherein the edge is substantially parallel to the attenuation axis.
10. The method of claim 9 wherein the dither axis y′ and the effective attenuation axis x′ are related to the switching axis y and the attenuation axis x through a rotation matrix A of the type:
A
=
[
cos
θ
-
M
sin
θ
(
sin
θ
)
/
M
cos
θ
]
wherein θ is an angle that is not equal to 0° or 90° or some integer multiple of 90° and M is not equal to 1.
11. The method of claim 10 , wherein the value of M is a ratio of an x-axis optical magnification to a y-axis optical magnification of the spectral channel.
12. The method of claim 9 , wherein the combination of rotations about the switching axis x and the attenuation axis y is chosen such that the resulting insertion loss exhibits an attenuation of about 10 dB with respect to a peak coupling insertion loss and a ripple of about 0.5 dB or less.
13. The method of claim 9 , wherein the combination of rotations about the switching axis x and the attenuation axis y is chosen such that the resulting insertion loss exhibits an attenuation of about 15 dB with respect to a peak coupling insertion loss and a ripple of about 1 dB or less.
14. The method of claim 9 , further comprising implementing hitless switching with the optical apparatus.
15. The method of claim 14 , wherein implementing hitless switching includes substantially attenuating or blocking one or more of the spectral beams during switching of one or more of the spectral channels from an input port to an output port.
16. The method of claim 15 , wherein substantially attenuating or blocking one or more of the spectral beams during switching includes:
disengaging a servo control loop controlling the switching axis of channel micromirror;
rotating the micromirror about its attenuation axis by an amount required for blocking while the servo control loop is disengaged;
rotating the channel micromirror about its switching axis to reflect an input spectral channel to a first output port;
rotating the micromirror about its attenuation axis back to a normal optimal coupling condition, and
re-engaging the servo control loop controlling the switching axis to control the micromirror to achieve optimum coupling efficiency.